Interferometric fibre optic sensors rely on the existence of optical signals travelling along two or more paths which combine at a certain point. The variation in phase delay between the optical signals results in cyclic fluctuation in the optical intensity at the optical path junction as measured by an optical detector such as a photo diode.
The magnitude of the fluctuation of intensity due to the interference in relation to the intensities of the separate optical signals is determined by their coherence and the accuracy with which their polarization axes are aligned at the path junction.
Coherence is assured by arranging that the optical signals in the separate paths originate from a common source, typically a laser, whose output signal coherence length, or the propagation distance over which optical phase is maintained, exceeds the difference between the separate optical path lengths. Polarization control is usually achieved by the use of polarization-maintaining or birefringent fibre with well defined optical axes and careful alignment of these axes for the separate optical paths.
Interferometers can take numerous forms. In a Fabry-Perot interferometer the separate optical paths consist of different numbers of passes over the same physical path between partially reflecting mirrors. The Michelson configuration splits the light from the source into two physical paths terminating in mirrors. The signals are reflected and recombine at the original splitting point. In the Mach-Zehnder case the two separate paths recombine at a second junction.
In a fibre interferometer all of the optical paths are within optical fibre. The splitting and recombining points consist of optical fibre directional couplers.
An example of a sensing application is a sound transducer such as a hydrophone which comprises at least one coil of continuous optical fibre called the sensing coil with associated partially reflecting joints, couplers and possibly a second reference coil, where the exact configuration depends on the interferometer type in use. The precise optical path length along the sensing coil is sensitive to sound waves in water in which the transducer is placed. Where present, the second reference coil is insulated from the sound field and typically provides compensation for the effects of temperature changes, static pressure, acceleration, etc by reducing the effect of these influences on the optical path length difference. This difference is typically less than 1% of the actual path lengths in the two coils.
In the simple case of a fibre Mach-Zehnder interferometer where the incident light is fed through a fused biconical taper 2.times.2 fibre optic coupler and the output signals are combined at a second 2.times.2 coupler, there is a net 180.degree. phase shift between the outputs of the combining coupler. The interference fringes include a cyclic null condition at which sensitivity is diminished. This is commonly referred to as signal fading. To avoid this condition, it has been proposed (Sheem, J. Appl. Phys. 52(6), June 1981, 3865) to combine the sensor output signals through a biconical taper 3.times.3 coupler, thereby producing three signals which are not in antiphase. Alternative signal processing techniques have utilised two or all three of these signals from the output ports of the 3.times.3 coupler.
For many sensing applications, and in particular for hydrophone applications, a practical requirement is that the sensors can be arranged in a multiplexed linear array, using a small number of downlead and uplead fibres. The fundamental limitation on all fibre optic multiplexing schemes involving passive power splitting is the amount of optical power available from the source and the efficiency with which this can be shared between all sensors and returned to the detector.
Many multiplexing schemes have been proposed, involving all manner of time-division, frequency division, and other techniques to separate the signals from the individual sensors. However, all schemes, which to date have generally involved single mode fibre only, have the same overall loss constraints. These are imposed by the symmetric splitting loss characteristics of the couplers which dictate that splitting loss must be the same in both directions. It is an object of the invention to overcome these present limitations on the wider application of interferometric sensors in multiplexed fibre optic networks.